“Thousands of species of microbes can live a single spoonful of soil. Although microbiologists have been naming species of bacteria for well over a century, they’ve described only a tiny fraction of the Earth’s single-celled diversity.

We can acquire many of these bacteria from our environment: from dirt, handshakes, doorknobs, and computer keyboards. Yet our inner menagerie is not simply a random sampling of the microbial traffic flowing around us. Many of these species have adapted to our human habitat over millions of years. They’ve evolved tricks for finding food in our nooks and crannies. Our own bodies have adapted to them as well, able to recognize them as friends rather than enemies. And it’s possible that some members of the human microbiome have evolved intimate ways to get from one generation to the next.”